Reheating and Primordial Gravitational Waves in Generalized Galilean Genesis

TL;DR

This paper explores how gravitational particle production reheats the universe after Galilean genesis and predicts a distinctive blue spectrum of primordial gravitational waves, which could be tested by future high-frequency detectors.

Contribution

It provides a detailed analysis of gravitational reheating and gravitational wave spectrum in generalized Galilean genesis within Horndeski theory, offering new predictions for high-frequency gravitational waves.

Findings

Reheating occurs via gravitational particle production at the transition from genesis.

The primordial gravitational wave spectrum is strongly blue, with  imes f^3 at high frequencies.

Amplitude of gravitational waves can reach  imes 10^{-12} at 100 MHz, testable in future experiments.

Abstract

Galilean genesis is an alternative to inflation, in which the universe starts expanding from Minkowski with the stable violation of the null energy condition. In this paper, we discuss how the early universe is reheated through the gravitational particle production at the trasition from the genesis phase to the subsequent phase where the kinetic energy of the scalar field is dominant. We then study the consequences of gravitational reheating after Galilean genesis on the spectrum of primordial gravitational waves. The resultant spectrum is strongly blue, and at high frequencies $\Omega_{\rm gw}\propto f^3$ in terms of the energy density per unit logarithmic frequency. Though this cannot be detected in existing detctors, the amplitude can be as large as $\Omega_{\rm gw}\sim 10^{-12}$ at $f\sim 100\,$MHz, providing a future test of the genesis scenario. The analysis is performed within the framework of generalized Galilean genesis based on the Horndeski theory, which enables us to derive generic formulas.